The over-all purpose of the experiments proposed in this application is to determine the role of intrarenal peritubular Starling forces in regulating renal salt and water excretion in heart failure and different types of renal disease. Abnormal salt and water retention occur in congestive heart failure and in the nephrotic syndrome. In some forms of renal disease, particularly interstitial nephritis, salt wasting in the urine may occur. The mechanisms of these disturbances are not fully understood. We propose to examine peritubular colloid oncotic pressure and hydrostatic pressure changes, flow rate in peritubular capillaries, and distribution of renal blood flow within the kidney by use of micropuncture techniques in rats with either heart failure or kidney disease. Hormonal factors, which may be responsible for either renal hemodynamic changes or direct effects on tubular transport, will be sought by carrying out cross-perfusion experiments between two rats, one with either heart or renal disease and the other normal. Micropuncture studies will be performed during the cross-circulation procedure on the normal rat to measure tubular function and peritubular Starling factors.